Solder-coated ball and method for manufacture thereof, and method for forming semiconductor interconnecting structure

ABSTRACT

A solder ball  50  according to the present invention includes a spherical core  2  and a solder layer  4 , which includes Sn and Ag and which is provided so as to wrap the core  2  up. The amount of water contained in the solder layer  4  is 100 μl/g or less when represented by the amount of water vapor in standard conditions.

TECHNICAL FIELD

The present invention relates to a solder ball for use as aninput/output terminal for a BGA or any other semiconductor device and amethod of making the ball.

BACKGROUND ART

As computer-related equipment has had its performance further enhancedand its sizes further reduced and as information network equipment hasbecome even more popular these days, it becomes increasingly necessaryto realize even higher-density mounting with a printed circuit board foruse in those types of equipment. In the past, a quad flatpack package(QFP) with lead terminals around its component would often be used as amember to realize high-density surface mounting. Recently, however, aball grid array (BGA), which is relatively small in size and which cancope with multiple-pin applications, is used more and more often. TheBGA may also be used as a spacer member when a quartz oscillator and atemperature-compensating IC are stacked one upon the other.

As shown in FIGS. 2(a) and 2(b), a ball grid array (BGA) is an LSIpackage in which solder balls 50 are bonded onto the lower surface of anLSI chip with an interposer 62 interposed between them. The solder balls50 are arranged in matrix on one surface of the interposer 62, and areused as input/output terminals for the package. Each of these solderballs 50 is a tiny sphere with a diameter of about 0.1 mm to about 1.0mm, and may be obtained by forming a solder layer on the surface of ametallic ball, for example.

If the solder layer of a lead-tin based material is deposited by anelectroplating technique, then voids may be created in the solder layerwhile the solder balls are being heated, melted and bonded onto the padsof the interposer, which is a serious problem. The reason is that oncethose voids have been created, the interposer and the solder balls areeither connected defectively or misaligned from each other, thusaffecting the reliability of the BGA.

The applicant of the present application discovered that those voidswere created by the hydrogen gas that was absorbed in the solder layerbeing formed by the electroplating technique, and could be minimized byreducing the absorption of that hydrogen gas. Based on this discovery,the applicant of the present application disclosed a method forminimizing the creation of voids by reducing the quantity of hydrogengas absorbed into the solder layer with the ion concentrations of leadand tin in the plating solution and the current density during theelectroplating process controlled (see Japanese Patent ApplicationLaid-Open Publication No. 10-270836, pages 2 and 3, in particular).

In recent years, solder with lead is being replaced with solder with nolead (which is also called “Pb-free solder”). As the Pb-free solder, anSn—Ag based solder or an Sn—Ag—Cu based solder is used, for example.

DISCLOSURE OF INVENTION

When a solder ball, including an Sn—Ag based solder layer, was made byan electroplating technique and heated and melted, voids were alsocreated as in the solder ball including the lead-tin based solder layerdescribed above. As a result of extensive researches, the presentinventors discovered that those voids were not created by the hydrogengas but another factor, which was unique to the Sn—Ag based solders aswill be described later.

In order to overcome the problems described above, objects of thepresent invention are to provide a solder ball that includes an Sn—Agbased solder layer, in which the creation of voids is minimized whilethe solder layer is being heated and melted, a method of making such aball, and a method of making a semiconductor interconnect structure.

A solder ball according to the present invention includes a sphericalcore and a solder layer, which includes Sn and Ag and which is providedso as to wrap the core up. The amount of water contained in the solderlayer is 100 μl/g or less when represented by the amount of water vaporin standard conditions, whereby the problems described above areovercome.

The solder layer may include an Sn—Ag alloy.

The solder layer may include a first metal layer, which is provided soas to wrap the core up, and a second metal layer, which is provided soas to wrap the first metal layer up, and one of the first and secondmetal layers may include Sn and the other metal layer may include Ag.

The core is preferably made of Cu, Al or a resin.

The solder layer preferably includes 0.5 mass % to 4.0 mass % of Ag.

The solder layer preferably includes Cu, Sn and Ag.

The solder layer preferably includes 3.5 mass % of Ag.

A method of making a solder ball according to the present inventionincludes the steps of: preparing a spherical core; forming a platinglayer, including Sn and Ag, by an electroplating technique such that theplating layer wraps the core up; heating the core with the platinglayer, thereby keeping the plating layer molten for a predeterminedperiod of time; and solidifying the molten plating layer, thereby makinga solder layer, whereby the problems described above are overcome.

The step of forming the plating layer may include the step of forming analloy plating layer including Sn and Ag.

The step of forming the plating layer may include the step of forming anadditional plating layer including Ag.

The step of forming the plating layer may include the steps of forming afirst plating layer, including Sn, such that the first plating layerwraps the core up, and forming a second plating layer, including Ag,such that the second plating layer also wraps the core up.

The solder layer may include Cu, Sn and Ag.

The solder layer preferably includes 0.5 mass % to 4.0 mass % of Ag.

The solder layer preferably includes 3.5 mass % of Ag.

Another solder ball according to the present invention is preferablymade by one of the methods described above.

Another method of making a solder ball according to the presentinvention includes the steps of preparing a spherical core and forming asolder layer, including Sn and Ag, such that the solder layer wraps thecore up. The step of forming the solder layer includes the step offorming a first solder layer, including an Sn—Ag alloy, by anelectroplating process that uses a plating solution including 10 g/l to25 g/l of tris(3-hydroxypropyl)phosphine, 15 g/l to 25 g/l of Snorganosulfonate, 0.3 g/l to 1.5 g/l of Ag organosulfonate, 50 g/l to 100g/l of organic sulfonic acid, and ammonia. The first solder layerincludes 0.5 mass % to 2.5 mass % of Ag, whereby the problems describedabove are overcome.

The plating solution preferably further includes 3 g/l to 12 g/l ofthiourea.

The step of forming the solder layer may further include the step offorming a second solder layer including Ag.

The second solder layer may be formed by an electroplating process, anevaporation process or a colloidal process.

The second solder layer is preferably formed by the electroplatingprocess and preferably has a thickness of at most 0.5 μm.

The solder layer preferably includes 3.0 mass % to 4.0 mass % of Ag.

The first solder layer preferably has a thickness of 3 μm to 50 μm.

The core is preferably made of Cu, Al or a resin.

The solder layer preferably includes 3.5 mass % of Ag.

The core preferably has a diameter of 0.05 mm to 1 mm.

Another solder ball according to the present invention is preferablymade by one of the methods described above.

A method of making a semiconductor interconnect structure according tothe present invention includes the steps of: preparing solder balls byone of the methods described above; preparing a board on which pads of aconductive material are arranged; putting and heating the solder ballson the pads, thereby turning the solder layer into a molten solderlayer; and solidifying the molten solder layer, whereby the problemsdescribed above are overcome.

Another solder ball according to the present invention includes aspherical core and a solder layer, which includes Sn and Ag and which isprovided so as to wrap the core up. The solder layer includes a firstsolder layer made of an Sn—Ag alloy. The first solder layer includes 0.5mass % to 2.5 mass % of Ag. And the amount of water contained in thesolder layer is 100 μl/g or less when represented by the amount of watervapor in standard conditions, whereby the problems described above areovercome.

The solder layer may further include a second solder layer, which isprovided so as to wrap up the first solder layer, and the second solderlayer preferably includes Ag and preferably has a thickness of at most0.5 μm.

The solder layer preferably includes 3.0 mass % to 4.0 mass % of Ag.

The first solder layer preferably has a thickness of 3 μm to 50 μm.

The core is preferably made of Cu, Al or a resin.

The solder layer preferably includes 3.5 mass % of Ag.

The core preferably has a diameter of 0.05 mm to 1 mm.

A semiconductor device according to the present invention preferablyincludes one of the solder balls described above.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) and 1(b) are cross-sectional views of solder balls accordingto first and second preferred embodiments of the present invention.

FIGS. 2(a) and 2(b) are respectively a perspective view and across-sectional view of a BGA that uses solder balls according to thefirst or second preferred embodiment of the present invention.

FIGS. 3(a) and 3(b) illustrate a method of making a semiconductorinterconnect structure according to the present invention.

FIGS. 4(a), 4(b) and 4(c) show how to spot voids.

FIGS. 5(a) and 5(b) are photographs that were taken of the firstspecific example and the first comparative example, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

In order to figure out why those voids were created while a solder ball,including an Sn—Ag based solder layer made by an electroplating process,was being heated and melted, the present inventors analyzed the gasemitted from the solder layer being heated and melted. As a result, thepresent inventors discovered that most of the gas emitted was watervapor. Based on this discovery, we acquired the following knowledge.

The water vapor, which was the main component of the emitted gas, wasproduced due to the vaporization of water, which was trapped in thesolder layer being formed by the electroplating process, during theprocess step of heating and melting. That is to say, the water vapor wasemitted from the solder layer being heated and melted, thereby creatingthose voids. Also, the water (i.e., that component to produce the watervapor under the heat) was trapped in the solder layer mainly because ofthe presence of Ag in the solder layer. Thus, it is believed to bebecause a hydrolytic product of Ag (such as Ag(OH)) was generated duringthe electroplating process.

Based on this knowledge, the present inventors acquired the basic ideaof the present invention to be described below.

Embodiment 1

FIG. 1 is a cross-sectional view of a solder ball 50 according to afirst preferred embodiment of the present invention. As shown in FIG. 1,the solder ball 50 includes a spherical core 2 and a solder layer 4,which includes Sn and Ag and which is provided so as to wrap up the core2. The solder layer 4 may include either a single layer as shown in FIG.1(a) or multiple layers as shown in FIG. 1(b). This solder layer 4 iscontrolled such that the amount of water contained in the solder layer 4is 100 μl/g or less when represented by the amount of water vapor instandard conditions.

In this solder ball 50, the solder layer 4 is controlled so as tocontain a sufficiently small amount of water as just described. Thus,the number of voids to be created while the solder layer 4 is beingheated and melted can be reduced significantly. As will be describedlater for specific examples of the present invention, the presentinventors confirmed via experiments that if the amount of watercontained in the solder layer 4 was controlled to this value or less,the decrease in the bond strength of the solder balls 50, misalignment,and other defects could be reduced sufficiently.

The “amount of water” is supposed herein to be measured by the followingmethod using a thermal desorption spectrometer (TDS) EMD-WA 100S(produced by ESCO. Ltd). Specifically, solder balls are put in anatmosphere that has been evacuated to a pressure of 2×10⁻⁶ Pa or lessand the temperature is raised from room temperature to 600° C. at a rateof 0.5° C./sec. In the meantime, the masses of gases produced aremeasured component by component with a quadrupole mass spectrometer.Regarding a gas component with a mass number of 18 as water, its totalquantity is obtained and then converted into a volume in standardconditions. The volume is divided by the mass of the solder layer 4 toobtain the amount of water (μl/g). It should be noted that the mass ofthe solder layer 4 was calculated by subtracting the mass of the core 2from that of the solder ball 50. The mass of this solder layer 4 was theaverage of the masses of approximately 100 samples. The masses of thoseapproximately 100 solder balls 50 and cores 2 were measured withprecision scales.

The solder balls 50 may be used as input/output terminals for BGAs andchip size packages (CSPs). FIG. 2 illustrates an exemplary BGA withsolder balls 50. Specifically, FIGS. 2(a) and 2(b) are respectively aperspective view and a cross-sectional view of the BGA 70. As shown inFIGS. 2(a) and 2(b), the BGA 70 includes an interposer 62, asemiconductor chip 64 mounted on one side of the interposer 62, and aplurality of solder balls 50 bonded on the other side thereof. Thesolder balls 50 are arranged in matrix on the surface of the interposer62 as shown in FIG. 2(a). The semiconductor chip 64 is encapsulated witha resin 66 and is electrically connected to the solder balls 50 by wayof metal wires 68 and interconnects 69, which are provided in theinterposer 62.

In the solder balls 50 of this preferred embodiment, the number of voidsto be created in the heated and melted state can be reducedsignificantly as described above. Thus, the defective connection andmisalignment, which might otherwise occur while the solder balls 50 arebeing fixed onto the interposer 62, can be minimized. As a result, thereliability of the BGA can be increased.

Hereinafter, the solder layer 4, of which the amount of water iscontrolled to 100 μl/g or less when represented by the amount of watervapor in standard conditions, will be described more fully.

The solder layer 4 may be a single plating layer including an Sn—Agalloy as shown in FIG. 1(a).

Alternatively, the solder layer 4 may also have a multilayer structureconsisting of a plurality of metal layers as shown in FIG. 1(b).Specifically, in that case, the solder layer 4 consists of a first metallayer 6, which is provided so as to wrap up the core 2, and a secondmetal layer 8, which is provided so as to wrap up the first metal layer6. One of the first and second metal layers 6 and 8 is a layer includingSn, while the other layer is a layer including Ag. Thus, even if thesolder layer 4 has a multilayer structure, soldering is realized (atleast in the bonded state) substantially in the same way as in thesituation where the solder layer 4 is made of an Sn—Ag alloy. It shouldbe noted that when the solder layer 4 has such a multilayer structure,the composition of the solder layer 4 can be controlled by adjusting thethicknesses of respective layers that make up the solder layer 4.

If the solder layer 4 has a multilayer structure as shown in FIG. 1(b),then the thicknesses of the first and second metal layers 6 and 8 aredetermined according to the desired composition ratio of solder. Also,the Sn-containing layer and the Ag-containing layer may be provided inany order as the first and second metal layers 6 and 8. However, it ispreferable that one of the two layers with the higher oxidationresistance is provided as the outer layer (i.e., the second metal layer8). Accordingly, when the solder layer 4 is made up of an Sn layer andan Ag layer, for example, the Ag layer is preferably provided as thesecond metal layer 8.

The mass percentage of Ag in the solder layer 4 is appropriatelydetermined according to the desired composition of the solder.Typically, the mass percentage of Ag contained is preferably 0.5 mass %to 4.0 mass %.

The core 2 may be made of Cu, for example. In that case, Cu diffusesfrom the core 2 into the solder layer 4 being heated, and Sn and Ag,included in the solder layer 4, and that Cu become the respectiveconstituent materials of the solder. That is to say, an Sn—Ag—Cu basedsolder is obtained.

If the core 2 is made of Cu, then the mass percentage of Ag included inthe solder layer 4 is preferably set to about 2 mass % to 4 mass %, morepreferably about 3.5 mass %. This is because as long as the masspercentage of Ag included in the solder layer 4 falls within this range,a ternary eutectic reaction of Sn—Ag—Cu occurs and a single meltingpoint of about 216° C. is obtained when the solder layer 4 is heated.Also, this melting point of about 216° C. is lower than that of a binaryeutectic Sn—Ag (about 221° C.). The melting point was supposed to be theonset temperature of a DTA curve that was measured at a temperature riserate of 2° C./min (i.e., melting start temperature).

It should be noted that if the solder layer has a eutectic composition,various advantages are achieved. For example, in the molten state, thesolder layer exhibits high flowability and guarantees good workefficiency. Plus, the solidified solder has such highly uniformcomposition and texture as to exhibit high mechanical strength, shearstrength, tensile strength and shock resistance. That is why such asolder layer with a eutectic composition is preferably used.

However, the material of the core 2 does not have to be Cu.Alternatively, the core 2 may also be made of either a metal such as Alor a resin. If the core 2 is made of a resin, then a layer of Ni or anyother suitable metal is preferably formed on the surface of the core 2by an electroless plating technique, for example, and then the solderlayer 4 is preferably deposited thereon by an electroplating technique.

Hereinafter, an exemplary method of making the solder ball 50 will bedescribed.

In a first method, a plating layer is dehydrated by heating and meltingit.

First, a spherical core 2 is prepared. Next, a plating layer, includingSn and Ag, is deposited thereon by an electroplating technique so as towrap up the core 2.

The plating layer may be formed by electroplating an Sn—Ag alloy.Alternatively, the plating layer may also be formed by electroplating anSn—Ag alloy (i.e., making a first plating layer) and then electroplatingAg (i.e., making a second plating layer). As another alternative, theplating layer may also be formed by electroplating Sn (i.e., making afirst plating layer) and then electroplating Ag (i.e., making a secondplating layer). There is a big difference in standard electrodepotential between Sn and Ag. Accordingly, if an Sn—Ag alloy iselectroplated on an industrial basis, then the plating conditions needto be controlled, and the plating solution needs to be managed, withhigh precision. In contrast, if the plating layer is made up of aplating layer including Sn and a plating layer including Ag, then nosuch high precision control or management is required. Consequently, theelectroplating process can be carried out more easily.

Next, the core 2, on which the plating layer (consisting of either asingle layer or multiple layers) has been deposited, is heated, therebykeeping the plating layer molten in a predetermined amount of time.

This heating and melting process step is carried out by putting thesolder ball 50 on a surface with low solder wettability (e.g., on astainless steel or ceramic substrate) and keeping the ball 50 heated toa prescribed temperature in a predetermined amount of time within aninert atmosphere of Ar, for example, of which the pressure was set equalto the atmospheric pressure. The heating temperature is defined severaltens ° C. higher than the melting point of the final material of thesolder layer 4. For example, if the core 2 is made of Cu, the platinglayer is made of an Sn—Ag alloy, and the mass percentage of Ag includedin the plating layer is about 3.5 mass % (where the melting point (i.e.,the ternary eutectic point) of the materials of the solder layer 4 is216° C.), then the solder ball 50 is heated to about 240° C. Thepredetermined amount of time is preferably 10 to 30 minutes.

By heating the plating layer and keeping it molten in a predeterminedamount of time as described above, the water trapped in the platinglayer during the electroplating process can be removed. As a result, thesolder ball 50, in which the amount of water contained in the solderlayer 4 is controlled to 100 μl/g or less when represented by the amountof water vapor in standard conditions, can be obtained.

In a second method, the solder layer 4 has a multilayer structureconsisting of two metal layers of Sn and Ag as shown in FIG. 1(b) andthe Ag layer is formed by a non-electroplating technique, e.g., anevaporation process. As described above, if a metal layer including Agis formed by an electroplating technique, water will be trapped in thatmetal layer. Thus, the Ag layer is formed by a non-electroplatingtechnique. Consequently, the solder ball 50, in which the amount ofwater contained in the solder layer 4 is controlled to the above valueor less, can be obtained.

In the following description, various interconnect structures, in whichsolder balls may be used for an element or device including asemiconductor chip at least, will be collectively referred to herein as“semiconductor interconnect structures”. Such a semiconductorinterconnect structure may be made by the following method, for example.

First, as shown in FIG. 3(a), solder balls 50 and a desired substrate20, on which the solder balls 50 will be bonded, are prepared. Thesubstrate 20 may be used as an interposer for a BGA (see FIG. 2) or aCSP. On the principal surface of the substrate 20, pads 18 of conductivematerials are provided. Each of those pads 18 may be a stack of a Culayer 12, a Ni plating layer 14 and an Au plating layer 16, for example.Next, the solder balls 50 on the pads 18 are heated, thereby melting thesolder layer 4 as shown in FIG. 3(b), where the molten solder layer isidentified by the reference numeral 4A. Then, the molten solder layer 4Ais cooled, solidified, and thereby bonded onto the pads 18. Byperforming these process steps, a semiconductor interconnect structureis formed.

In this semiconductor interconnect structure, the solder balls 50 arebonded to the substrate 20 with so strongly that misalignment and otherinconveniences are rarely caused. As a result, a highly reliablesemiconductor interconnect structure can be provided.

Hereinafter, specific examples of the present invention will bedescribed. The solder balls of this preferred embodiment are preferablyformed by an electroplating process. However, the electroplating processdoes not have to be carried out as described below but may be performedfollowing a known procedure. For instance, an alkane sulfonic acid bath(see Japanese Patent Application Laid-Open Publications Nos. 8-13185 and12-34593, for example) will be used as a plating solution for plating anSn—Ag alloy in the following specific examples. Alternatively, agluconic acid-iodide bath (see Japanese Patent Application Laid-OpenPublication No. 10-36995, for example) or a tartaric acid bath (seeSurface Technology 49.758 (1998), for example) may also be used.

EXAMPLE 1

In a solder ball 50 representing a first specific example of the presentinvention, the solder layer 4 is a single Sn—Ag alloy layer.Hereinafter, a method of making the solder ball 50 of the first specificexample will be described.

First, a spherical copper core with a diameter of 0.8 mm ispre-processed with a 17.5% HCl aqueous solution at room temperature forone minute (process step (a)). Next, the core is washed (immersed forone minute and rinsed for one minute) with pure water at roomtemperature (process step (b)). Subsequently, the core is immersed in anorganic acid at room temperature for 30 seconds (process step (c)).Thereafter, the core is plated with a plating solution including tinmethanesulfonate (24 g/l of Sn), silver methanesulfonate (1.4 g/l ofAg), sulfonic acid, hydroxycarboxylic acid, organophosphorus compoundand thiourea (300° C.) at a current density of 0.30 A/dm², therebyforming an Sn—Ag alloy plating layer (with a thickness of 35 μm) (step(d)) Then, the plating layer is washed (immersed for one minute andrinsed for one minute) with pure water at room temperature (process step(e)). These process steps (a) through (e) are carried out within abarrel container. Thereafter, the solder ball is picked out of thebarrel container, washed (immersed for two minutes and rinsed for twominutes) with pure water at room temperature (process step (f)) and thendried at 60° C. for 10 minutes (process step (g)). Finally, this solderball is dehydrated by heating it at 240° C. for 10 minutes within an Aratmosphere at the atmospheric pressure.

In this manner, a solder ball representing the first specific example(including 3.5 mass % of Ag) was obtained.

EXAMPLE 2

In a solder ball 50 representing a second specific example of thepresent invention, the solder layer 4 consists of an Sn plating layer 6and an Ag evaporation layer 8. Hereinafter, a method of making thesolder ball 50 of the second specific example will be described.

First, a spherical copper core with a diameter of 0.5 mm ispre-processed with a 17.5% HCl aqueous solution at room temperature forone minute (process step (a)). Next, the core is washed (immersed forone minute and rinsed for one minute) with pure water at roomtemperature (process step (b)). Subsequently, the core is immersed in anorganic acid at room temperature for 30 seconds (process step (c)).Thereafter, the core is plated with a plating solution including tinmethanesulfonate (60 g/l of Sn) (40° C.) at a current density of 0.30A/dm², thereby forming an Sn plating layer (with a thickness of 34.2 μm)(step (d)). Then, the plating layer is washed (immersed for one minuteand rinsed for one minute) with pure water at room temperature (processstep (e)). These process steps (a) through (e) are carried out within abarrel container. Thereafter, the solder ball is picked out of thebarrel container, washed (immersed for two minutes and rinsed for twominutes) with pure water at room temperature (process step (f)) and thendried at 60° C. for 10 minutes (process step (i)). Next, the pressure isreduced to 1×10⁻⁴ Pa, Ar is introduced as an inert gas, and an Ag film(with a thickness of 0.8 μm) is deposited at a pressure of 1×10⁻² Pa byan ion plating process (process step (g)). Then, the solder ball iswashed again (immersed for two minutes and rinsed for two minutes) withpure water at room temperature (process step (h)) and then dried at 60°C. for 10 minutes (process step (i)).

In this manner, a solder ball representing the second specific example(including 3.7 mass % of Ag) was obtained.

EXAMPLE 3

In a solder ball 50 representing a third specific example of the presentinvention, the solder layer 4 consists of an Sn plating layer 6 and anAg plating layer 8. Hereinafter, a method of making the solder ball 50of the third specific example will be described.

First, a spherical copper core with a diameter of 0.3 mm ispre-processed with a 17.5% HCl aqueous solution at room temperature forone minute (process step (a)). Next, the core is washed (immersed forone minute and rinsed for one minute) with pure water at roomtemperature (process step (b)). Subsequently, the core is immersed in anorganic acid at room temperature for 30 seconds (process step (c)).Thereafter, the core is plated with a plating solution including tinmethanesulfonate (60 g/l of Sn) (40° C.) at a current density of 0.30A/dm², thereby forming an Sn plating layer (with a thickness of 10μm)(step (d)). Then, the plating layer is washed (immersed for oneminute and rinsed for one minute) with pure water at room temperature(process step (e)). Next, the Sn plating layer is further plated with aplating solution including silver iodide (20 g/l of Ag) (40° C.) at acurrent density of 0.10 A/dm², thereby forming an Ag plating layer (witha thickness of 0.24 μm) (step (f)). Then, the plating layer is washedwith pure water at room temperature (process step (g)). These processsteps (a) through (g) are carried out within a barrel container.Thereafter, the solder ball is picked out of the barrel container,washed (immersed for two minutes and rinsed for two minutes) with purewater at room temperature (process step (h)) and then dried at 60° C.for 10 minutes (process step (i)). In this manner, a solder ball 50representing the third specific example (including 3.6 mass % of Ag) wasobtained.

In the solder ball 50 of the third specific example, the Ag platinglayer was relatively thin, and therefore, the amount of water containedin the solder layer could be reduced sufficiently even without heating,melting and dehydrating the solder layer. However, if the Ag platinglayer is so thick that the amount of water contained in the solder layerexceeds 100 μl/g when represented by the amount of water vapor instandard conditions, the process step of dehydrating the solder layer byheating and melting it may be carried out after the step (i) as in thefirst specific example described above. Then, the amount of water can bereduced sufficiently.

COMPARATIVE EXAMPLES 1, 2 AND 3

For the purpose of comparison, solder balls representing first, secondand third comparative examples were made. In each of the solder balls ofthe first through third comparative examples, the solder layer 4 was asingle Sn—Ag alloy layer and was not dehydrated by heating and meltingit.

The solder ball of the first comparative example was made by the samemethod as that of the first specific example described above except thatthe solder layer was not dehydrated by heating and melting it.

The solder ball of the second comparative example was made by the samemethod as that of the first comparative example except that a sphericalcopper core with a diameter of 0.5 mm was used. The solder ball of thesecond comparative example included 3.7 mass % of Ag.

The solder ball of the third comparative example was made by the samemethod as that of the first comparative example except that a sphericalcopper core with a diameter of 0.3 mm was used and that the solder layerhad a thickness of 10 μm. The solder ball of the third comparativeexample included 3.6 mass % of Ag.

Evaluation

To evaluate the solder balls of the specific and comparative examples,the amounts of water contained in respective solder balls werecalculated. Also, those solder balls were heated and melted to count thenumber of voids created and measure the maximum diameter thereof, andwere also photographed. Furthermore, those solder balls were subjectedto a bonding test.

The maximum diameter and the number of voids were obtained in thefollowing manner. First, as shown in FIG. 4(a), the solder ball was puton a Cu substrate 30 with a flux 32 interposed on the principal surfacethereof. Next, as shown in FIG. 4(b), the solder ball was heated at 250°C. for 10 seconds, thereby melting the solder layer 4 (i.e., turning itinto molten solder 4A). Thereafter, as shown in FIG. 4(c), the Cu coreportion was removed from the solder ball. The fracture, exposed byremoving the Cu core, was photographed from over it, the number of voidscreated on the fracture was counted, and the maximum diameter thereofwas measured. Only the number of voids with diameters of at least 10 μmwas counted.

The bonding test was carried out in the following manner. Specifically,100 solder balls were put on the Cu substrate 30 as shown in FIG. 4(a).Next, as shown in FIG. 4(b), the solder layer 4 was heated and meltedand then cooled and solidified, thereby bonding the solder layer 4 ontothe substrate 30. This heating and melting process step was carried outby loading the substrate 30, on which the solder balls were arranged, inan oven having an internal temperature of 250° C. and replaced with anitrogen atmosphere for 10 seconds. Thereafter, the substrate 30 wasunloaded from the oven and then cooled by itself to room temperature.

Then, it was counted how many solder balls came off (or dropped from)the substrate 30 among the 100 balls obtained by the method describedabove.

Results

The photographs of the first specific example and the first comparativeexample are shown in FIGS. 5(a) and 5(b), respectively.

The amounts of water contained, the numbers of voids created and itsmaximum diameter measured for the first to third specific examples andthe first to third comparative examples and the numbers of balls droppedin the bonding test are shown in the following Table 1: TABLE 1 Amountof Maximum void Number of balls water diameter Number dropped (in 100(μl/g) (μm) of voids balls) Example 1 30 — 0 0 Example 2 65 — 0 0Example 3 80 — 0 0 Comp. Ex. 1 190 80 12 1 Comp. Ex. 2 180 60 15 2 Comp.Ex. 3 180 55 9 1

As can be seen from Table 1, each of the solder balls of the firstthrough third specific examples contained an amount of water of lessthan 100 μl/g, whereas each of the solder balls of the first throughthird comparative examples contained an amount of water exceeding 100μl/g and in the range of 180 μl/g to 190 μl/g. It should be noted thatthe solder ball of the first specific example contained an amount ofwater of 190 μl/g (corresponding with that of the first comparativeexample) before dehydrated by heating and melting it but contained anamount of water of 30 μg/l after dehydrated by heating and melting it.

As can be seen from FIG. 5(a) and Table 1, no voids were detected at allin the first through third specific examples, while 9 to 15 voids withdiameters of 55 to 80 μm were detected per mm² in the first throughthird comparative examples. Thus, the present inventors confirmed thatthe creation of voids was minimized effectively in the solder ball ofthis specific example containing an amount of water of less than 100μl/g. As also can be seen from Table 1, no bonding failures were spottedat all in the first through third specific examples, while few bondingfailures were spotted in the first through third comparative examples.Consequently, the solder ball of this specific example turned out to bebondable even more firmly.

Embodiment 2

In a solder ball according to a second preferred embodiment of thepresent invention, the amount of water contained in the solder layer 4,including Sn and Ag, is controlled to 100 μl/g or less when representedby the amount of water vapor in standard conditions as in the solderball of the first preferred embodiment described above. Thus, just likethe first preferred embodiment, the number of voids to be created whilethe solder layer 4 is being heated and melted can be reducedsignificantly. Also, as will be described later for specific examples ofthe present invention, the present inventors confirmed via experimentsthat if the amount of water contained in the solder layer 4 wascontrolled to this value or less, the decrease in the bond strength ofthe solder balls 50, misalignment, and other defects could be reducedsufficiently.

The second preferred embodiment is mainly characterized in that thesolder layer includes an Sn—Ag alloy solder layer to be made by anelectroplating process using a predetermined plating solution.

Hereinafter, the solder ball 50 of the second preferred embodiment willbe described with reference to FIGS. 1(a) and 1(b).

As long as the solder ball of this second preferred embodiment includesat least one Sn—Ag alloy solder layer, the solder layer may consist ofeither just one layer as shown in FIG. 1(a) or a plurality of layers asshown in FIG. 1(b).

If the solder layer 4 consists of a single layer, then the solder ball50 includes a core 2 and a solder layer 4 made of an Sn—Ag alloy asshown in FIG. 1(a).

On the other hand, if the solder layer 4 consists of multiple layers,then the solder ball 50 includes a solder layer 4, which is made up of afirst metal layer 6 and a second metal layer 8 that is provided so as towrap up the first metal layer 6. For example, the first metal layer 6may be a solder layer made of an Sn—Ag alloy, while the second metallayer 8 may be a solder layer of Ag. The first and second metal layers 6and 8 will be referred to herein as a “first solder layer” 6 and a“second solder layer” 8, respectively.

Even if the solder layer 4 has a multilayer structure as shown in FIG.1(b), soldering is realized (at least in the bonded state) substantiallyin the same way as in the situation where the solder layer 4 is made ofan Sn—Ag alloy as shown in FIG. 1(a). It should be noted that when thesolder layer 4 has such a multilayer structure, the composition of thesolder layer 4 can be controlled by adjusting the thicknesses ofrespective layers that make up the solder layer 4.

Hereinafter, a method of making the solder ball 50 shown in FIG. 1(a)will be described.

First, a spherical core 2 is prepared.

Next, a solder layer 4 made of an Sn—Ag alloy is deposited by anelectroplating process so as to wrap up the core 2. As the platingsolution, a solution including 10 g/l to 25 g/l oftris(3-hydroxypropyl)phosphine, 15 g/l to 25 g/l of Sn organosulfonate,0.3 g/l to 1.5 g/l of Ag organosulfonate, 50 g/l to 100 g/l of organicsulfonic acid, and ammonia is used. Ammonia is added to adjust the PH ofthe solution. The PH is preferably controlled within the range of 3.5 to5.0. As the Sn organosulfonate, Ag organosulfonate, and organic sulfonicacid, Sn methanesulfonate, Ag methanesulfonate and methanesulfonic acidare preferably used, respectively, as will be described later forspecific examples of the present invention. More preferably, the platingsolution further includes 3 g/l to 12 g/l of thiourea. This platingsolution is described in detail in Japanese Patent Application Laid-OpenPublication No. 2000-34593.

The solder layer 4 is made of this plating solution so as to include atmost 2.5 mass % of Ag. The electroplating process is preferably carriedout with the current density controlled to the range of 0.1 A/dm² to 0.6A/dm² and with the temperature of the plating solution controlled to therange of 20° C. to 30° C.

By performing these process steps, the solder ball 50 can be obtainedwith the amount of water contained in the solder layer 4 controlled to100 μl/g or less when represented by the amount of water vapor instandard conditions. If the electroplating process is carried out withthe predetermined plating solution described above, the amount of watercontained in the solder layer 4 can be reduced sufficiently withoutconducting any special-purpose treatment separately.

To make the solder layer 4 include more than 2.5 mass % of Ag, thesolder layer 4 preferably has a multilayer structure as shown in FIG.1(b). The solder layer 4 including more than 2.5 mass % of Ag has afiner crystal structure when melted. As a result, the bond strength ofthe solder ball 50 can be increased.

Such a solder ball 50, of which the solder layer has a multilayerstructure, may be made by forming the first solder layer 6 by theelectroplating process described above and then the second solder layer8 including Ag. When the solder layer 4 has a multilayer structure, thefirst solder layer 6 preferably includes at least 0.5 mass % of Ag.Then, the surface roughness of the first solder layer 6 can be reducedsufficiently. As a result, the first and second solder layers 6 and 8can contact with each other even more closely. The second solder layer 8of Ag may be made by an electroplating process, an evaporation processor a colloidal process, for example.

If the second solder layer 8 is formed by an electroplating process,then the thickness of the second solder layer 8 is defined to be 0.5 μmor less. As described above, it would be mainly because of the presenceof Ag component that water is trapped in a solder layer formed by anelectroplating process. Thus, by making the Ag layer sufficiently thin,the amount of water trapped in the solder layer can be reduced.

On the other hand, if the second solder layer 8 is formed by anon-electroplating method, then the thickness thereof does not have tobe set to 0.5 μm or less. However, when the thickness is equal to orsmaller than this value, the solder layer in the molten state is likelyto have a uniform composition, and therefore, the creation of abnormalgrains can be minimized.

According to the method described above, a solder ball 50, of which thesolder layer 4 includes more than 2.5 mass % of Ag and in which theamount of water contained in the solder layer 4 is controlled to 100μl/g or less when represented by the amount of water vapor in standardconditions, can be obtained.

The thicknesses of the first and second solder layers 6 and 8 aredetermined according to the desired composition ratio of solder.Optionally, it is not impossible to make the solder ball 50 shown inFIG. 1(b) such that the first solder layer 6 is an Ag layer and thesecond solder layer 8 is an Sn—Ag alloy layer. However, it is preferablethat one of the two layers with the higher oxidation resistance isprovided as the outer layer (i.e., the second solder layer 8).Specifically, there is a mixture of grains with various compositions inthe Sn—Ag alloy layer. Accordingly, if a solder ball, of which thesecond solder layer 8 is an Sn—Ag alloy layer, were left in the air fora long time, then its surface would be easily oxidized, corroded anddeformed, and its wettability and bond strength would decrease while thesolder ball is soldered. That is why it is preferable that the firstsolder layer 6 is an Sn—Ag alloy layer and the second solder layer 8 isan Ag layer.

Also, the solder layer 4 preferably includes 3.0 mass % to 4.0 mass % ofAg. This is because if the mass percentage of Ag included in the solderlayer 4 is approximately 3.5 mass %, then an Sn—Ag binary eutecticreaction will occur and a single melting point of about 221° C. can beobtained when the solder layer is heated. As will be described later, ifthe solder layer has a eutectic composition, various advantages areachieved. For example, the bond strength thereof can be increasedsufficiently. Also, if the mass percentage of Ag exceeded 4.0 mass %,then Ag₃Sn plate-like initial crystals (or needle-like initial crystals)with excessively large grain sizes of several tens of μm would becrystallized due to the heat to create cracks in the solder layer. Thatis why the mass percentage of Ag is preferably at most 4.0 mass % (seeKatsuaki Suganuma, “Lead-free Soldering Technology—Trump ofEnvironmentally Friendly Mounting”, Kogyo Chosakai Publishing Inc., Jan.20, 2001).

The core 2 may be made of Cu, for example. In that case, Cu diffusesfrom the core 2 into the solder layer 4 being heated, and Sn and Ag,included in the solder layer 4, and that Cu become the respectiveconstituent materials of the solder. That is to say, an Sn—Ag—Cu basedsolder is obtained.

If the core 2 is made of Cu, then the mass percentage of Ag included inthe solder layer 4 is preferably set to 2.0 mass % to 4.0 mass %, morepreferably about 3.5 mass %. This is because as long as the masspercentage of Ag included in the solder layer 4 falls within this range,a ternary eutectic reaction of Sn—Ag—Cu occurs and a single meltingpoint of about 216° C. is obtained when the solder layer 4 is heated.Also, this melting point of about 216° C. is lower than that of a binaryeutectic Sn—Ag (about 221° C.). The melting point was supposed to be theonset temperature of a DTA curve that was measured at a temperature riserate of 2° C./min (i.e., melting start temperature).

It should be noted that if the solder layer has a eutectic composition,various advantages are achieved. For example, in the molten state, thesolder layer exhibits high flowability and guarantees good workefficiency. Plus, the solidified solder has such highly uniformcomposition and texture as to exhibit high mechanical strength, shearstrength, tensile strength and shock resistance. That is why such asolder layer with a eutectic composition is preferably used.

However, the material of the core 2 does not have to be Cu.Alternatively, the core 2 may also, be made of either a metal such as Alor a resin. If the core 2 is made of a resin, then a layer of Ni or anyother suitable metal is preferably formed on the surface of the core 2by an electroless plating technique, for example, and then the solderlayer 4 is preferably deposited thereon by an electroplating technique.

The diameter of the core 2 typically falls within the range of 0.05 mmto 1 mm. As long as the size of the core 2 falls within this range,sufficiently high bond strength is achieved during the solderingprocess. Also, the balls can be bonded onto a substrate, for example, ata rather high density. The solder layer 4 or 6 made of the Sn—Ag alloytypically has a thickness of 3 μm to 50 μm.

Just like the solder ball 50 of the first preferred embodiment describedabove, the solder ball 50 of this second preferred embodiment may alsobe used as an input/output terminal for a BGA (see FIG. 2) or a CSP, forexample. By using this solder ball 50, the number of voids to be createdwhile the solder ball 50 is being heated and melted can be reducedsignificantly. Thus, defective connection or misalignment can beminimized when the solder balls 50 are fixed onto the interposer 62. Asa result, the reliability of the BGA can be increased.

In addition, by using the solder balls 50 of the second preferredembodiment, a highly reliable semiconductor interconnect structure, inwhich the solder balls 50 are soldered with the substrate 20 with sostrongly that misalignment or any other defect rarely happens, can beprovided as in the first preferred embodiment. Such a semiconductorinterconnect structure may be made by the same method as that alreadydescribed for the first preferred embodiment with reference to FIG. 3.

Hereinafter, specific examples of the present invention will bedescribed.

EXAMPLE 4

In a solder ball 50 representing a fourth specific example of thepresent invention, the solder layer 4 is a single Sn—Ag alloy layer.Hereinafter, a method of making the solder ball 50 of the fourthspecific example will be described.

First, a spherical copper core 2 with a diameter of 0.85 mm is prepared.Meanwhile, a solution including 15 g/l oftris(3-hydroxypropyl)phosphine, Sn methanesulfonate (24 g/l of Sn), Agmethanesulfonate (0.7 g/l of Ag), 60 g/l of methanesulfonate, and 5 g/lof thiourea is prepared, and an ammonia salt is added thereto, therebypreparing a plating solution with the PH controlled to 4.0.

Next, the core is plated with this plating solution at a current densityof 0.30 A/dm² and at a bath temperature of 30° C. using Sn as an anodeelectrode, thereby forming an Sn—Ag alloy plating layer 4 (with athickness of 35 μm) on the surface of the copper core 2. Thiselectroplating process is carried out within a barrel container.

In this manner, a solder ball representing the fourth specific example(of which the solder layer 4 includes 1.8 mass % of Ag) was obtained.

EXAMPLE 5

In a solder ball 50 representing a fifth specific example of the presentinvention, the solder layer 4 is also a single Sn—Ag alloy layer as inthe fourth specific example. Hereinafter, a method of making the solderball 50 of the fifth specific example will be described.

First, a spherical copper core 2 with a diameter of 0.60 mm is prepared.Meanwhile, a solution including 20 g/l oftris(3-hydroxypropyl)phosphine, Sn methanesulfonate (24 g/l of Sn), Agmethanesulfonate (0.95 g/l of Ag), 70 g/l of methanesulfonate, and 5 g/lof thiourea is prepared, and an ammonia salt is added thereto, therebypreparing a plating solution with the PH controlled to 4.0.

Next, the core is plated with this plating solution at a current densityof 0.30 A/dm² and at a bath temperature of 20° C. using Sn as an anodeelectrode, thereby forming an Sn—Ag alloy plating layer 4 (with athickness of 20 μm) on the surface of the copper core 2. Thiselectroplating process is carried out within a barrel container.

In this manner, a solder ball representing the fifth specific example(including 2.4 mass % of Ag) was obtained.

EXAMPLE 6

In a solder ball 50 representing a sixth specific example of the presentinvention, the solder layer 4 consists of an Sn—Ag alloy layer 6 and anAg layer 8. Hereinafter, a method of making the solder ball 50 of thesixth specific example will be described.

First, a spherical copper core 2 with a diameter of 0.50 mm is prepared.Meanwhile, a solution including 13 g/l oftris(3-hydroxypropyl)phosphine, Sn methanesulfonate (24 g/l of Sn), Agmethanesulfonate (0.4 g/l of Ag), and 50 g/l of methanesulfonate isprepared, and an ammonia salt is added thereto, thereby preparing aplating solution with the PH controlled to 4.0.

Next, the core is plated with this plating solution at a current densityof 0.30 A/dm² and at a bath temperature of 30° C. using Sn as an anodeelectrode, thereby forming an Sn—Ag alloy layer 6 (with a thickness of10 μm) on the surface of the copper core 2. The alloy layer 6 includes1.0 mass % of Ag.

Then, an Ag layer 8 (with a thickness of 0.17 μm) is deposited on thealloy layer 6 by an electroplating process using a silver iodide platingbath. This electroplating process is carried out within a barrelcontainer.

In this manner, a solder ball representing the sixth specific examplewas obtained. In this solder ball, the solder layer 4 consists of theSn—Ag alloy layer 6 and the Ag layer 8 and includes 3.5 mass % of Ag.

For the purpose of comparison, solder balls representing the fourththrough sixth comparative examples to be described below were also made.

COMPARATIVE EXAMPLE 4

A solder ball representing a fourth comparative example is differentfrom the fourth specific example in that the electroplating process iscarried out with the following plating solution. Specifically, theplating solution for use in this fourth comparative example includes Snmethanesulfonate (20 g/l of Sn), Ag methanesulfonate (0.3 g/l of Ag),and 100 g/l of methanesulfonate and its PH is controlled to less than1.0.

The plating process is carried out under the same conditions as those ofthe fourth specific example, except that this plating solution is usedat a bath temperature of 25° C., thereby forming an Sn—Ag alloy platinglayer (with a thickness of 35 μm) on the surface of the copper core. Inthis manner, a solder ball representing the fourth comparative example(including 1.8 mass % of Ag) was obtained.

COMPARATIVE EXAMPLE 5

A solder ball representing a fifth comparative example is different fromthe fifth specific example in that the electroplating process is carriedout with the following plating solution. Specifically, the platingsolution for use in this fifth comparative example includes Sn sulfate(17 g/l of Sn), Ag sulfate (0.4 g/l of Ag), and 200 g/l of potassiumiodide and its PH is controlled to 9.0.

The plating process is carried out under the same conditions as those ofthe fifth specific example, except that this plating solution is used ata bath temperature of 25° C., thereby forming an Sn—Ag alloy platinglayer (with a thickness of 20 μm) on the surface of the copper core. Inthis manner, a solder ball representing the fifth comparative example(including 2.4 mass % of Ag) was obtained.

COMPARATIVE EXAMPLE 6

A solder ball representing a sixth comparative example is different fromthe sixth specific example in that the Sn—Ag alloy is plated using thefollowing plating solution. Specifically, the plating solution for usein this sixth comparative example includes Sn methanesulfonate (18 g/lof Sn), Ag methanesulfonate (0.2 g/l of Ag), and 100 g/l ofmethanesulfonate and its PH is controlled to less than 1.0.

The plating process is carried out under the same conditions as those ofthe sixth specific example, except that this plating solution is used ata bath temperature of 25° C., thereby forming an Sn—Ag alloy platinglayer (with a thickness of 10 μm) on the surface of the copper core. Thealloy plating layer includes 1.0 mass % of Ag.

Then, as in the sixth specific example, an Ag layer (with a thickness of0.17 μm) is deposited on the alloy plating layer using a silver iodideplating bath.

In this manner, a solder ball representing the sixth comparative examplewas obtained. In this solder ball, the solder layer consists of theSn—Ag alloy plating layer and the Ag layer and includes 3.5 mass % ofAg.

Evaluation

To evaluate the solder balls of the specific and comparative examples,the amounts of water contained in respective solder balls werecalculated. Also, those solder balls were heated and melted to count thenumber of voids created and measure the maximum diameter thereof.Furthermore, those solder balls were subjected to a bonding test.

The maximum diameter and the number of voids were obtained in thefollowing manner. First, as shown in FIG. 4(a), the solder ball was puton a Cu substrate 30 with a flux 32 interposed on the principal surfacethereof. Next, as shown in FIG. 4(b), the solder ball was heated at 250°C. for 10 seconds, thereby melting the solder layer 4 (i.e., turning itinto molten solder 4A). Thereafter, as shown in FIG. 4(c), the Cu coreportion was removed from the solder ball. The fracture, exposed byremoving the Cu core, was photographed from over it, the number of voidscreated on the fracture was counted, and the maximum diameter thereofwas measured. Only the number of voids with diameters of at least 10 μmwas counted.

The bonding test was carried out in the following manner. Specifically,100 solder balls were put on the Cu substrate 30 as shown in FIG. 4(a).Next, as shown in FIG. 4(b), the solder layer 4 was heated and meltedand then cooled and solidified, thereby bonding the solder layer 4 ontothe substrate 30. This heating and melting process step was carried outby loading the substrate 30, on which the solder balls were arranged, inan oven having an internal temperature of 250° C. and replaced with anitrogen atmosphere for 10 seconds. Thereafter, the substrate 30 wasunloaded from the oven and then cooled by itself to room temperature.

Then, it was counted how many solder balls came off (or dropped from)the substrate 30 among the 100 balls obtained by the method describedabove.

Results

The amounts of water contained, the numbers of voids created and itsmaximum diameter measured for the fourth to sixth specific examples andthe fourth to sixth comparative examples and the numbers of ballsdropped in the bonding test are shown in the following Table 2: TABLE 2Amount of Maximum void Number of balls water diameter Number dropped (in100 (μl/g) (μm) of voids balls) Example 4 50 — 0 0 Example 5 70 — 0 0Example 6 80 — 0 0 Comp. Ex. 4 200 80 14 1 Comp. Ex. 5 200 70 16 1 Comp.Ex. 6 200 60 12 1

As can be seen from Table 2, each of the solder balls of the fourththrough sixth specific examples contained an amount of water of lessthan 100 μl/g, whereas each of the solder balls of the fourth throughsixth comparative examples contained an amount of water of 200 μl/g.

As can be seen from Table 2, no voids were detected at all in the fourththrough sixth specific examples, while 12 to 16 voids with diameters of60 to 80 μm were detected per mm² in the fourth through sixthcomparative examples. Thus, the present inventors confirmed that thecreation of voids was minimized effectively in the solder ball of thisspecific example containing an amount of water of less than 100 μl/g. Asalso can be seen from Table 2, no bonding failures were spotted at allin the fourth through sixth specific examples, while a bonding failurewas spotted in the fourth through sixth comparative examples.Consequently, the solder ball of this specific example turned out to bebondable even more firmly.

INDUSTRIAL APPLICABILITY

The present invention provides a solder ball that includes an Sn—Agbased solder layer, in which the creation of voids is minimized whilethe solder layer is being heated and melted, and also provides a methodof making such a ball. A solder ball according to the present inventioncan be used effectively as an input/output terminal for a BGA or a CSP,for example.

1. A solder ball comprising a spherical core, and a solder layer, whichincludes Sn and Ag and which is provided so as to wrap the core up,wherein the amount of water contained in the solder layer is 100 μl/g orless when represented by the amount of water vapor in standardconditions.
 2. The solder ball of claim 1, wherein the solder layerincludes an Sn—Ag alloy.
 3. The solder ball of claim 1, wherein thesolder layer includes a first metal layer, which is provided so as towrap the core up, and a second metal layer, which is provided so as towrap the first metal layer up, and wherein one of the first and secondmetal layers includes Sn and the other metal layer includes Ag.
 4. Thesolder ball of claim 1, wherein the core is made of Cu, Al or a resin.5. The solder ball of claim 1, wherein the solder layer includes 0.5mass % to 4.0 mass % of Ag.
 6. The solder ball of claim 1, wherein thesolder layer includes Cu, Sn and Ag.
 7. The solder ball of claim 6,wherein the solder layer includes 3.5 mass % of Ag.
 8. A method ofmaking a solder ball, the method comprising the steps of: preparing aspherical core; forming a plating layer, including Sn and Ag, by anelectroplating technique such that the plating layer wraps the core up;heating the core with the plating layer, thereby keeping the platinglayer molten for a predetermined period of time; and solidifying themolten plating layer, thereby making a solder layer.
 9. The method ofclaim 8, wherein the step of forming the plating layer includes the stepof forming an alloy plating layer including Sn and Ag.
 10. The method ofclaim 9, wherein the step of forming the plating layer includes the stepof forming an additional plating layer including Ag.
 11. The method ofclaim 8, wherein the step of forming the plating layer includes thesteps of: forming a first plating layer, including Sn, such that thefirst plating layer wraps the core up, and forming a second platinglayer, including Ag, such that the second plating layer also wraps thecore up.
 12. The method of claim 8, wherein the solder layer includesCu, Sn and Ag.
 13. The method of claim 12, wherein the solder layerincludes 0.5 mass % to 4.0 mass % of Ag.
 14. The method of claim 12,wherein the solder layer includes 3.5 mass % of Ag.
 15. A solder ballmade by the method of claim
 8. 16. A method of making a solder ball, themethod comprising the steps of preparing a spherical core, and forming asolder layer, including Sn and Ag, such that the solder layer wraps thecore up, wherein the step of forming the solder layer includes the stepof forming a first solder layer, including an Sn—Ag alloy, by anelectroplating process that uses a plating solution including 10 g/l to25 g/l of tris (3-hydroxypropyl) phosphine, 15 g/l to 25 g/l of Snorganosulfonate, 0.3 g/l to 1.5 g/l of Ag organosulfonate, 50 g/l to 100g/l of organic sulfonic acid, and ammonia, and wherein the first solderlayer includes 0.5 mass % to 2.5 mass % of Ag.
 17. The method of claim16, wherein the plating solution further includes 3 g/l to 12 g/l ofthiourea.
 18. The method of claim 16, wherein the step of forming thesolder layer further includes the step of forming a second solder layerincluding Ag.
 19. The method of claim 18, wherein the second solderlayer is formed by an electroplating process, an evaporation process ora colloidal process.
 20. The method of claim 19, wherein the secondsolder layer is formed by the electroplating process and has a thicknessof at most 0.5 μm.
 21. The method of claim 18, wherein the solder layerincludes 3.0 mass % to 4.0 mass % of Ag.
 22. The method of claim 16,wherein the first solder layer has a thickness of 3 μm to 50 μm.
 23. Themethod of claim 16, wherein the core is made of Cu, Al or a resin. 24.The method of claim 16, wherein the solder layer includes 3.5 mass % ofAg.
 25. The method of claim 16, wherein the core has a diameter of 0.05mm to 1 mm.
 26. A solder ball made by the method of one of claims 16 to25.
 27. A method of making a semiconductor interconnect structure, themethod comprising the steps of: preparing solder balls by the method ofone of claim 8 and 16; preparing a board on which pads of a conductivematerial are arranged; putting and heating the solder balls on the pads,thereby turning the solder layer into a molten solder layer; andsolidifying the molten solder layer.
 28. A solder ball comprising aspherical core, and a solder layer, which includes Sn and Ag and whichis provided so as to wrap the core up, wherein the solder layer includesa first solder layer made of an Sn—Ag alloy, and wherein the firstsolder layer includes 0.5 mass % to 2.5 mass % of Ag, and wherein theamount of water contained in the solder layer is 100 μl/g or less whenrepresented by the amount of water vapor in standard conditions.
 29. Thesolder ball of claim 28, wherein the solder layer further includes asecond solder layer, which is provided so as to wrap up the first solderlayer, and wherein the second solder layer includes Ag and has athickness of at most 0.5 μm.
 30. The solder ball of claim 29, whereinthe solder layer includes 3.0 mass % to 4.0 mass % of Ag.
 31. The solderball of claim 28, wherein the first solder layer has a thickness of 3 μmto 50 μm.
 32. The solder ball of claim 28, wherein the core is made ofCu, Al or a resin.
 33. The solder ball of claim 30, wherein the solderlayer includes 3.5 mass % of Ag.
 34. The solder ball of claim 28,wherein the core has a diameter of 0.05 mm to 1 mm.
 35. A semiconductordevice including the solder ball of one of claims 1, 15, 26 and 28.